Electrolytic treatment of metal surfaces



I March 12, 1963 R. w. GURRY 3,081,238 ELECTROLYTIC TREATMENT OF METAL SURFACES Filed Sept. 3, 1 958 United States Patent 3,081,238 ELETROLYTIC TREATMENT OF METAL SURFACES Robert W. Garry, Berwyn, Pa, assignor to Quaker Chemical Corporation, a corporation of Pennsylvania Filed Sept. 3, 1958, Ser. No. 758,747 4 Claims. (Cl. 204-34) This invention relates to a process of applying a durable protective coating to the surface of a ferrous metal, such as iron, steel and the blackplate used in the canning and container industries, by cathodic treatment in a chromic acid solution. The purpose of the coating is (1) to enhance the corrosion resistance of the metal, (2) to provide a base for an adherent organic film, e.g., paint, lacquer or enamel, and (3) to reduce the underfihn corrosion of the painted surface.

This application is a continuation-in-part of application Serial No. 665,140, filed June 12, 1957, now abandoned.

his an object of this invention toprovide an effective means of applying such a corrosion-resistant, enamel-adhering coating to'the metal surface. A'further object of this invention is to provide a rapid process for applying i this coating, in particular one adapted to continuous operation in a high-speed production line. Another object is to provide a continuous process that is easy to operate and simple to control. For example, it is highly desirable from a production standpoint that the various treating 4 baths be stable and long lived, and thus not subject to :frequent adjustment of composition or dumping. Still another object of the invention is to provide a process for which the installation and operating costs of the required apparatus'is low, so that, in combination with the Y high line speed and the eflectiveness of the resulting prod uct, a competitive advantage in the trade will result. 7

It .is often desirable to protect ferrous metal surfaces against the formation of objectionable corrosion products during further processing, storage or actual use. It is also often desirable to increase the paint-bonding ability of ferrous metal surfaces and the corrosion resistance of the painted surface. All of these purposes can be accomplishedby application to the metal surface of a suitable film, whichimparts bothcorrosion resistance and 1 paint-bonding properties. Hitherto certain types of films have been found effective, notably phosphates and chromates, These are usually applied by contact with, or electrolysis in, aqueous solutions of a wide variety of compositions.

The phosphate films used for metal protection without further coating are generally relatively thick; although effective in'resisting corrosion, they are extremely slow of deposition, brittle and not applicable where there is to be any appreciable deformation of the metal such as bending, drawing, forming, etc. The thinner phosphate films, used as a base for paint, are not so subject to rupture from deformation and they are cheaper to apply; but they do not afford a degree of protection against corrosion that is often necessary.

Chromate films are usually thin, protective and adherent, although not always offering as good a' bond for paint as the phosphate type. They are applied under a very wide range-of conditions of composition, pH, temperature and current density. mate or dichroniate ion from either chromic acid or its alkali salts, the solutions usually contain an inorganic anion such as phosphate or borate, added as the acid or a salt. Other inorganic anions such a nitrate, chloride of fluoride-have been used as well as the complex fluorides, fluosilicate, fluoborate, etc. Sometimes organic acid anions are added. Often a portion of the hexavalent chromium in solution is reducedto the trivalent state by reaction with some organic material, for example sugar,

In addition to the chroreducing agents.

the cathodic treatment.

3,081,233 Patented Mar. 12, 1963 i'ice .glycerine, alcohol, or the like. Cathodic treatment in a bath containing chromic acid has also been suggested in Patent 1,827,247, but the conditions given fail to produce a satisfactory coating at a rate necessary for a modern continuous treating line.

I have found that a thin, adherent paint-bonding film imparting very superior corrosion resistance to steel strip can be applied at an extremely rapid rate by a process involving an activating acid pretreatment, rinsing, cathodic treatment in an aqueous bath containing only chromic acid, rinsing, and drying. Each of these steps is necessary and constitutes an integral part of the process as a whole.

By the process of this invention an electrolytic treatment as short as one second produces a coating with ex cellent corrosion resistance and enamel adherence. it will be apparent that the process of my invention is simple and easy to control. The chromic acid bath is chemically stable and has a long life, inasmuch as it contains no It is readily rejuvenated by the addition of chromium trioxide.

A unique feature of my process lies in the fact that rust spots on the original strip are reduced to metal during Rust on blackplate is an ever present difficulty and cannot be prevented commercially in any economical way. The commercial advantage of a process which negates the unsightly appearance and corrosion initiating tendencies of rust on strip is very great.

,No other known process for the chemical treatment of steel can accomplish this.

The protective film produced by the process of this invention is complex in nature and its chemical nature is not known definitely. It is an inorganic, non-metallic film which may contain hydrated chromic oxide with retained hexavalent chromium, possibly in the form of The appearance of the film may The protection of the base metal afforded by the film is of a very high order, much superior to that from films of comparable thickness cathodically applied in chromic acid baths in the presence of phosphate, borate, or trivalent chromium ion. When steel panels treate cathodically in chromic acid solution were coated with enamel, baked, scratched diagonally from corner to corner, and subjected to corrosion test by immersion for 18 hours in a solution containing 3% NaCl and 0.3% H 0 at F., the surface was extremely well inhibited against corrosion, including the scratches which were cut deeply into the base metal. The adhesion of the enamel was perfect after test, even immediately adjacent to the scratch. There was no underfilrn corrosion. Panels prepared in the same manner except with added phosphate, borate, or chromic ion in the electrolytic solution showed much more corrosion as well as some loss of adhesion. The enamel, applied by dipping and draining, or by roller coating so as to obtain an even coat, was a phenolic type baking enamel. The panels were cured for 10 minutes at 386 F.

Further details and advantages of my invention are given in the following explanation and accompanying drawing, wherein F-iGURE 1 illustrates diagrammatically an apparatus for carrying out my process continuously for treating a moving strip of blackplate'in accordance with my invention.

According to my invention the steel strip, if not already free from oil, grease and dirt is cleaned by any conventional method, such as cleaning in hot alkaline solution either with or without benefit of electrolytic action. Referring to FIGURE 1, the clean metal strip '10 is continuously uncoiled from the pay-oft reel 5, passed over guide roll 6, into tank 1 which contains an acid solution, preferably sulfuric acid, over lower guide roll EG'bGiWCfiIl a pair of squeegee or wringer rolls 7, and over guide roll 8.

After the acid pretreatment, the strip 10 continuously passes over guide roll 9, through chamber 2 where it is rinsed with Water from spray nozzles 14, over guide roll 31, then between additional water spray nozzles 14a, then between squeegee rolls 13 and over guide roll 12. The conditions of the acid pretreatment result in a mild action, but either ovcrtreating or undertreating is to be avoided. In a solution containing of sulfuric acid by weight, immersion for one second at 60 C. for 3 seconds at 40 C. is satisfactory, although immersion times varying from about one second to about five seconds may be used in baths containing from about 0.5% to about by weight and at temperatures varying from about 20 C. to about 90 C. In general, the immersion time will vary inversely with the acid concentration and the tem- .perature.

The strip 10 is then immediately given the cathodic treatment in a bath in tank 3 containing about-100 to about 500 g. CrO per liter. The current density may range from about 200 to about 800 amperes per sq. ft., and the time of thetreatment from about 1 second to about 5 seconds. The strip 10 continuously passes over metal roll connected to the negative pole of a DC. generator 16, then between spaced anodes 19 connected to the positive pole of the generator 16 and immersed in the electrolytic bath in tank 3, then over guide roll 32, also immersed in the bath, then upwardly between another pair of spaced anodes 19a, then between squeegee rolls 17 and over guide roll 150, also connected to the negative pole of the generator 16.

Although the electrolytic bath temperature is not a critical factor in the production of my protective surface film, I prefer to use a temperature falling in the range from about 20 C. to about 70 C., more specifically from about 45 C. to about 60 C. Below about 45 C. the electrolysis is ineffective in reducing rust, as heretofore described, and above about 60 C. the cost of heating the bath and difiiculties of handling the noxious fumes militate against the commercial utility of my process.

After the electrolytic treatment in tank 3, the strip 10 passes immediately and continuously over guide roll 21, between water spray nozzles 22 in chamber 4, over guide roll 24, between water spray nozzles 22a, then between squeegee rolls 23 and over guide rolls 30 and 31. The strip 10 then passes through a hot air dryer 35, over a guide roll 36 and is coiled up on take-up reel 37.

In carrying out the process of this invention, the activating acid pretreatment previous to the electrolytic bath is essential. If the strip is given no pretreatment, though initially clean and bright, the product is quite deficient in corrosion resistance, particularly when enamelled, as measured by the chloride-peroxide immersion test described above. At the completion of this test the rinsed and dried panels are inspected for rust, and the adhesion of the enamel is evaluated by applying pressure sensitive tape to the surface and quickly jerking it away. Deficient adhesion results in the rem-oval of. some enamel with the tape. Unpretreated panels, those given an alkaline pretreatment, and those given an oxidizing pretreatment either in solution, as in nitric acid or ammonium persulfate, ,or by heating at a somewhat elevated temper.- ature, are all appreciably deficient compared to those pretreated in sulfuric acid. The deficiency exhibits itself in a greatly increased number of rust spots and a loss of enamel adhesion. The electrolytic process of this invention requires an active oxide-free surface, such as that imparted to commercial blackplate by a mild treatment in sulfuric acid and rinsing. A more drastic acid treatment such as the usual pickling is again detrimental, presumably due to the carbides, carbon and non-metallic inclusions left in and on the roughened surface. Acids other than sulfuric may be used, but oxidation must be avoided and the electrolytic treatment must follow immediately. Exposure to the liquid of the electrolytic bath prior to the passage of current is detrimental to the product and must be avoided. Also, after electrolysis, the strip must be rinsed immediately. A delay results in a lowered corrosion resistance of the product.

The chromic acid bath of this invention operates at a very low pH, preferably less than unity. Alkali ions in small amounts can be tolerated, but major amounts are detrimental to corrosion resistance. For example, the bath cannot be made up from an alkali dichromate.

Higher temperatures and higher concentrations of chromic acid within the limits of this invention favor the reduction of rust on the metal strip during cathodic treatment as heretofore described.

In the manufacture of food containers, one side of the treated steel panel corresponding to the inside of the can is often enamelled to minimize the extent of attack by the can contents and the other side of the panel, to save the cost, is not. Thus, the outside of the can must have suitable protection against rusting during storage in the atmosphere. It has been found that a thin film of oil applied over the surface treated by the process of this-invention imparts to the surface an abnormally high degree of rust resistance. Any one of a number of oils can be used, the dictate being largely one of cost. Cottonseed oil performs very well and can be applied by spraying, preferably with electrostatic aid, or by dipping or roller coating with appropriate solvent dilution so that the final oil film is very thin. Natural or synthetic resin solutions or emulsions may also be used.

If such 'an oiling step is added to the process of this invention, it is inserted between the hot air drier and the take-up reel. Also, in the event that oil, grease or dirt is present on the initial blackplate strip, this is removed by an alkaline cleaning unit, either electrolytic or otherwise, inserted in the line between the pay-off reel and the pretreating bath.

The following is an illustrative example of carrying out the process of my invention. Clean blackplatc is passed from the uncoiler into an aqueous bath in tank 1 of 5% sulfuric acid at 50 C. where it receives a 1 second treatment. It is then rinsed with water sprays and immediately immersed as a cathode in an electrolytic bath in tank 3 containing 200 g. per liter of chromium trioxide free of impurities at a temperature of 50 C. using a current density of 400 amp. per sq. ft. for 2 seconds. The treated blackplate strip is then immediately rinsed again by water sprays and is passed between squeegee rolls. It is dried by warm air and coiled.

The chromic acid bath does not deteriorate during use other than through loss of chromic acid due to the cathode reaction and drag-out. Only the periodic addition of chromic acid is necessary in order to maintain the initial concentration of the bath.

This invention is applicable to iron and steel, particularly the blackplate used by the container industry. All component parts of the process are very rapid and lend themselves to a high speed production line. The treated strip is found to have superior durable corrosion resistance both before and after the application of a tightly adhering film of paint, varnish, lacquer, or enamel. The electrolytic bath contains no reducing agents and is therefore chemically stable and long lived. The control of this bathis simple and the process as a whole is easy to operate. The low cost of all processing chemicals, the extremely high production line speed, and the high quality of the product of my process result in a great commercial advantage.

I claim:

1. The process of reducing rust on and imparting to a continuously moving strip of ferrous metal a durable rust resistant coating resistant for 18 hours to corrosion by an aqueous solution containing 3 sodium chloride and 6.3% hydrogen peroxide at a temperature of 75 C. which consists essentially of continuously activating the surfaces of said strip in a dilute aqueous bath of sulfuric acid for a few seconds, immediately rinsing the metal surfaces with water, immediately thereafter immersing the moving ferrous metal surfaces in an electrolytic bath as a cathode, said bath consisting essentially of water and 100 to 500 gms. CrO per liter and having a temperature of between 45 and 60 C., subjecting the metal surfaces to a current density between 200 and 800 amp. per sq. ft. for l to 5 seconds, then immediately rinsing the coated metal surfaces in water for a few seconds and drying the coated ferrous metal.

2. The process of reducing rust on and imparting to a continuously moving strip of blackplate a durable rust resistant coating resistant for 18 hours to corrosion by an aqueous solution containing 3% sodium chloride and 0.3% hydrogen peroxide at a temperature of 75 C. which consists essentially of subjecting the moving surfaces of the blackplate to the activating action of an aqueous bath of 5% sulfuric acid at about 60 C. for 1 second, immediately rinsing the blackplate with water, immediately thereafter cathodically treating the moving blackplate strip in an electrolytic bath consisting essentially of water and 200 gms. pure CrO per liter at a temperature of 50 C. and current density of about 400 amp. per sq. ft. for 2 seconds, immediately rinsing the coated surfaces of the blackplate with water, then squeegeeing the surfaces of the blackplate, and drying the coated ferrous metal.

3. The process of reducing rust on and imparting to a continuously moving strip of ferrous metal a durable rust resistant coating resistant for 18 hours to corrosion by an aqueous solution containing 3% sodium chloride and 0.3% hydrogen peroxide at a temperature of 75" C.

which consists essentially of continuously activating the surfaces of said strip in a dilute bath of sulfuric acid for a few seconds, immediately thereafter depositing upon said moving surfaces an inorganic, nonmetallic coating by subjecting said surfaces to cathodic treatment in an aqueous bath at a temperature of between from about 20 to about C. consisting essentially of water and 1.00 to 500 gm. per liter of CrO at a current density of 260 to 800 amp. per sq. ft. for 1 to 5 seconds, and thereafter immediately rinsing said coated surfaces and drying the coated ferrous metal.

4. The process of reducing rust on and imparting to a continuously moving strip of ferrous metal a durable rust resistant coating resistant for 18 hours to corrosion by an aqueous solution containing 3% sodium chloride and 0.3% hydrogen peroxide at a temperature of C. which consists essentially of continuously activating the surfaces of said strip in a dilute bath of sulfuric acid for a few seconds, immediately rinsing the treated strip, immediately thereafter cathodically treating the moving strip in an electrolytic bath consisting essentially of water and 200 gms. pure CrO per liter at a temperature of 50 C. and current density of about 400 amp. per sq. ft. for 2 seconds, immediately rinsing the coated surfaces of the strip with water, then squeegeeing the surfaces of the strip, and drying the coated ferrous metal.

References Cited in the file of this patent UNITED STATES PATENTS 1,745,912 Richardson Feb. 4, 1930 1,827,247 Mason Oct. 13, 1931 2,392,871 Wick Ian. 15, 1946 2,733,199 Wick Ian. 31, 1956 2,780,592 Wick Feb. 5, 1957 2,812,297 Stareck et a1. Nov. 5, 1957 

1. THE PROCESS OF REDUCING RUST ON AND IMPARTING TO A CONTINUOUSLY MOVING STRIP OF FERROUS METAL A DURABLE RUST RESISTANT COATING RESISTANT FOR 18 HOURS TO CORROSION BY AN AQUEOUS SOLUTION CONTANING 3% SODIUM CHLORIDE AND 0.3% HYDROGEN PEROXIDE AT A TEMPERATURE OF 75* C. WHICH CONSISTS ESSENTIALLY OF CONTINUOUSLY ACTIVATING THE SURFACES OF SAIS STRIP IN A DILUTE AQUEOUS BATH OF SULFURIC ACID FOR A FEW SECONDS, IMMEDIATELY RINSING THE METAL SURFACES WITH WATER, IMMEDIATELY THEREAFTER IMMERSING THE MOVING FERROUS METAL SURFACES IN AN ELECTROLYTIC BATH AS A CATHODE, SAID BATH CONSISTING ESSENTIALLY OF WATER AND 100 TO 500 GMS. CRO3 PER LITER AND HAVING A TEMPERATURE OF BETWEEN 45 AND 60* C., SUBJECTING THE METAL SURFACES TO A CURRENT DENSITY BETWEEN 200 AND 800 AMP. PER SQ. FT. FOR 1 TO 5 SECONDS, THEN IMMEDIATELY RINSING THE COATED METAL SURFACES IN WATER FOR A FEW SECONDS AND DRYING THE COATED FERROUS METAL. 